How to Test and Inspect a Used Piano

To determine the overall condition of a used piano you can do a simple visual inspection and some short physical tests. While these tests won't tell you everything you need to know, they will tell you whether a piano is even worth considering.

Look at the overall condition of the case, finish.
Bleaching/Cracking indicates that it's been directly exposed to the sun light - not good. Significant dents scratches on the bottom and bottom corners may indicate it was dropped or moved improperly. A very dark finish indicates the age (older finishes darken over time). Any water marks/lines are very bad - excessive moisture is the enemy of pianos.

Where is the piano sitting? Where has it been stored?
A piano in non climate controlled storage is subject to extremes of both moisture and temperature. This is very bad on a piano, it slowly - due to expansion and contraction - breaks apart the glued joints and warps the wood. Also a piano sitting along an outside wall is subject to different temperatures. The side facing the wall will often be hotter/cooler than the other side.

Are the keys even, side to side, and up and down.
Get down at eye-level to the top of the keys. Look to see that the tops of the keys are all even with each other. Pianos in poor condition will have an irregular, uneven keys. If it's a few keys, it may not be so bad. If whole sections are lower that may indicate a more serious problem. After looking at the tops of the keys, stand up and look down at the keys. Are the distances between the keys similar? Are some keys slanted, closer to their neighbors than others. Irregularities in the keys are indicators of possible problems, and a general indicator of the condition of the piano

Look inside the piano
Open the lid. It's located on the top of a piano. Look inside at the parts and the strings. Similar to the key inspection, the hammers (the striking part made of felt) should be even and very uniform in their spacing. The strings shouldn't be too rusted. The tuning pins (the metal plugs connecting the strings to the wood through the metal plate) should not be rusted. And, if you see some stains around these pins - looks like liquid stains - the piano may have been treated because of loose pins. That's bad news.